A water-absorbent resin is widely used in (a) sanitary articles such as disposable diapers, sanitary napkins, adult incontinence pads, (b) water-retaining agents for soils, (c) and the like. Such water-absorbent resins are produced and consumed in large quantities.
Conventionally, a method based on aqueous solution polymerization is known as a method for producing the water-absorbent resin. Known examples of such method are (i) a method in which a hydrogel obtained by polymerizing aqueous solution including a hydrophilic monomer is crushed while being stirred, (ii) a method in which a hydrogel obtained by carrying out static polymerization with respect to aqueous solution including a hydrophilic monomer is crushed, (iii) and the like. Among them, the latter method based on the static polymerization is advantageous in that, for example, a reaction device provided with a continuously conveyable endless belt enables continuous production with ease.
In the method based on the static polymerization, the hydrogel obtained by the polymerization is carried from the reaction device to a gel primary crusher or the like for example so that the hydrogel is crushed into pieces each of which has a predetermined size. However, the hydrogel is generally adhesive, so that the hydrogel is likely to adhere to the reaction device's surface which is in contact with monomer liquid (hereinafter, the surface is referred to also as “contact portion”). This results in various problems. For example, in case of carrying out the polymerization in the reaction device provided with a continuously conveyable endless belt, the hydrogel is likely to be entrapped, clogged, buckled, rolled in, or causes a similar problem, so that it is impossible to smoothly convey. Further, the hydrogel is extended which may result in cracks and severances. In this case, the cracked portion or the severed portion is likely to be rolled into a guide roll or a rotation roll etc., or the cracked portion or the severed portion is likely to be entrapped at a vicinity of an inlet of the gel primary crusher. Each of various problems brought about by adhesion of the hydrogel greatly decreases the productivity in producing the water-absorbent resin. Thus, improvement thereof is so desired.
As means for solving the problems brought about by the adhesion of the hydrogel, a technique for facilitating detachment of the hydrogel by spraying water between the contact portion and the hydrogel is proposed (see Japanese Unexamined Patent Publication No. 131218/2001 (Tokukai 2001-131218)). However, the hydrogel detached by spraying water has high water content, so that it is necessary to excessively dry the hydrogel thereafter. This causes lower productivity and disadvantage in terms of the production cost.
Further, as means for preventing the adhesion of the hydrogel, there is proposed means for carrying out electrolytic polishing with respect to the contact portion so that its surface roughness Rmax is not more than 3 μm and for always keeping a rear side of the contact portion cooled in stirring/polymerizing the monomer liquid (see Japanese Examined Patent Publication No. 15568/1994 (Tokukohei 6-15568)). However, in the static polymerization carried out by using the reaction device provided with a continuously conveyable endless belt, the polymerization is carried out in an unstirred state, so that the hydrogel is more likely to adhere to the contact portion. As a result, it is impossible to sufficiently prevent the adhesion even when the foregoing means is adopted. Moreover, in case where the foregoing means is adopted in the device provided with the endless belt, it is necessary to always keep the rear side of the contact portion cooled. This condition requires higher cost. In view of this, the foregoing means has disadvantage.
As another means for facilitating the detachment of the hydrogel, a material of the contact portion of the reaction device can be taken into consideration. For example, there is proposed a method in which a surfactant used as a general sheet detachment agent is applied to the contact portion of the reaction device (see Japanese Unexamined Patent Publication No. 155405/1986 (Tokukaisho 61-155405)). However, in this case, the surfactant adheres to the surface of the hydrogel, and the surfactant remains also on the water-absorbent resin obtained as a final product. The surfactant remaining on the water-absorbent resin decreases surface tension of liquid (urine or the like) absorbed in a sanitary product such as a disposable diaper for example. This results in such a problem that wet back of the absorbed liquid (urine or the like) increases.
As means which is free from the problem that the surface tension decreases, there is disclosed means for using fluororesin as the contact portion of the reaction device or for coating the contact portion with fluororesin (see Japanese Unexamined Patent Publication No. 155963/2004 (Tokukai 2004-155963), Japanese Examined Patent Publication No. 15481/1983 (Tokukosho 58-15481), and Unexamined Patent Publication No. 55002/1985 (Tokukaisho 60-55002)). However, when the inventors of the present invention carried out the means disclosed in each of Japanese Unexamined Patent Publication No. 155963/2004 (Tokukai 2004-155963), Japanese Examined Patent Publication No. 15481/1983 (Tokukosho 58-15481), and Unexamined Patent Publication No. 55002/1985 (Tokukaisho 60-55002) by using polytetrafluoroethylene (PTFE) which is a most general fluororesin, it was proved that the adhesion of the hydrogel was not sufficiently prevented. For example, in case of carrying out continuous production by using the reaction device provided with the continuously conveyable endless belt, the adhesion of the hydrogel hardly occurred in a short time. However, when the reaction device was operated for an extended period of time, the adhesion gradually occurred, which resulted in lower productivity. Moreover, in the water-absorbent resin obtained by using the PTFE as the fluororesin in the means disclosed in each of Japanese Unexamined Patent Publication No. 155963/2004 (Tokukai 2004-155963), Japanese Examined Patent Publication No. 15481/1983 (Tokukosho 58-15481), and Unexamined Patent Publication No. 55002/1985 (Tokukaisho 60-55002), a large amount of an extractable content is likely to be contained. When the water-absorbent resin containing a large amount of an extractable content is used in sanitary articles such as a disposable diaper for example, this causes a rough skin, so that it is desired to solve the problem brought about by the extractable content.
On the other hand, as the continuously conveyable reaction device based on the static polymerization, it is conventionally general to use an endless belt conveyer. In the reaction device provided with a continuously conveyable endless belt, it is preferable to use as the contact portion a flexible material such as a film or a sheet in view of a condition under which a strip-shaped belt having no end is moved in a loop shape. However, in this case, a driving tension at the time of conveyance is entirely exerted to the film or the sheet, so that the film or the sheet may be broken. Thus, it is difficult to carry out continuous production for an extended period of time. Moreover, also when a larger device is provided in consideration for the productivity (particularly, when the device is enlarged in a longitudinal direction), this raises a problem in terms of durability of the film or the sheet. Thus, it is desired to solve these problems in order to improve the productivity.